public int searchTree(final int levelsToTop, int alpha, final int beta) {
if (board.isDraw()) {
return scorer.drawValue();
}
if (levelsToTop == iterativeDepth) {
return scorer.staticScore(board);
}
final MoveContainer moves = moveContainers[levelsToTop];
final BoardHashEntry hashEntry = hashTable.get(board.getHashCode());
prioritizeHashEntry(hashEntry, moves);
board.makeNullMove();
board.generateValidMoves(moves);
if (moves.isEmpty()) {
return scorer.endOfGameValue(board.isInCheck(), levelsToTop);
}
for (int m = 0; m < moves.size(); m++) {
final Move move = moves.get(m);
board.makeMove(move);
final int childScore = -searchTree(levelsToTop + 1, -beta, -alpha);
board.undoMove();
if (childScore > alpha) {
// narrowing alpha beta window
alpha = childScore;
movePath.markMove(levelsToTop, iterativeDepth, m);
if (alpha >= beta) {
// pruned!
break;
}
}
}
unprioritizeHashEntry(hashEntry, moves);
hashTable.set(
board.getHashCode(),
alpha,
0,
movePath.getRaw(levelsToTop),
board.getMoveNumber(),
BoardHashEntry.ValueBounds.PV);
return alpha;
}
private static void unprioritizeHashEntry(
final BoardHashEntry entry, final MoveContainer moveContainer) {
if (entry != null) {
moveContainer.unprioritizeMove(entry.getBestMove());
}
}